mwifiex: correction in number of bitrates

In recent commit "mwifiex: correct bitrates advertised..", we have
removed 22Mbps and 72Mbps bitrates from supported bitrate array.
It means number of bitrates has reduced from 14 to 12.

Initialize ".n_bitrates" to array size instead of hardcoding it.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>